Microelectronics Guide: Semiconductor Physics, Diodes, and Transistors
Microelectronics is the science and engineering of devices on the microscopic scale — primarily transistors, diodes, and the integrated circuits built from them. The semiconductor industry has driven the most dramatic technological advancement in human history, from the first integrated circuit with a handful of transistors in 1958 to today’s chips containing tens of billions of transistors on a single die.
Understanding microelectronics means understanding how materials that are neither good conductors nor good insulators can be engineered to create switches and amplifiers of extraordinary capability. This knowledge is foundational for every other area of electronics — from analog circuit design to VLSI systems to power semiconductor devices.
Semiconductor Physics
The behavior of semiconductors is determined by their energy band structure. In a conductor, the valence band and conduction band overlap, allowing electrons to move freely. In an insulator, a large band gap separates the bands. In a semiconductor, the band gap is small enough that electrons can be promoted from the valence band to the conduction band by thermal energy, light, or an applied electric field.
Silicon is the dominant semiconductor material because of its mature manufacturing technology, excellent native oxide for insulation, and reasonable band gap of 1.12 electron volts. Germanium has higher carrier mobility but a narrower band gap that limits high-temperature operation. Gallium arsenide has higher electron mobility and a direct band gap that makes it ideal for optoelectronics and high-frequency devices.
Intrinsic silicon has equal numbers of electrons and holes at thermal equilibrium. Doping introduces impurity atoms that donate extra electrons (n-type) or accept electrons leaving holes (p-type). The doping concentration, from 10^13 to 10^21 atoms per cubic centimeter, determines the conductivity and the properties of the resulting devices.
Carrier Transport
Current in semiconductors results from the drift of carriers under an electric field and the diffusion of carriers from regions of high concentration to low concentration. Electron mobility is roughly three times hole mobility in silicon, meaning n-type devices are generally faster than p-type devices. Mobility decreases with increasing doping and temperature due to increased scattering.
The PN junction is the simplest semiconductor device and the building block for all others. When p-type and n-type materials are brought together, carriers diffuse across the junction, creating a depletion region with no free carriers. The built-in potential, about 0.7 volts for silicon, prevents further diffusion at equilibrium.
Diodes
A diode is a PN junction with electrical contacts. Under forward bias, the applied voltage reduces the built-in potential, allowing current to flow. Under reverse bias, the depletion region widens and only a tiny leakage current flows — until the reverse voltage exceeds the breakdown voltage, at which point current increases dramatically.
The current-voltage characteristic of an ideal diode follows the Shockley equation: I = Is(e^(qV/nkT) - 1), where Is is the saturation current, q is the electron charge, V is the applied voltage, n is the ideality factor, k is Boltzmann’s constant, and T is the temperature. The exponential relationship means that current increases by a factor of 10 for every 60 millivolts of forward voltage at room temperature.
Special-Purpose Diodes
Zener diodes are designed to operate in reverse breakdown at a precisely controlled voltage. They are used as voltage references and shunt regulators in power supplies. The breakdown voltage is set during manufacturing by controlling the doping concentration.
Schottky diodes use a metal-semiconductor junction instead of a PN junction. They have lower forward voltage drop, about 0.3 volts, and faster switching because they are majority-carrier devices with no stored charge. They are essential in switching power supplies and high-frequency rectifiers.
Varactor diodes use the voltage-dependent capacitance of a reverse-biased junction for tuning circuits. Light-emitting diodes convert electrical energy directly into light through electroluminescence. Photodiodes detect light by generating current from absorbed photons. These optoelectronic devices bridge the gap between electronics and photonics.
Bipolar Junction Transistors
The bipolar junction transistor consists of three semiconductor regions — emitter, base, and collector — arranged as either NPN or PNP. The base-emitter junction is forward biased, and the base-collector junction is reverse biased. Electrons injected from the emitter into the thin base region are swept across the base-collector junction by the electric field.
The transistor’s key characteristic is current gain: a small base current controls a much larger collector current. In the common-emitter configuration, the current gain is approximately the collector current divided by the base current. A typical gain of 100 means that a 10-microamp base current controls a 1-milliamp collector current, providing significant signal amplification.
The Early effect describes how the collector current increases with collector-emitter voltage because the base-collector depletion region widens, effectively reducing the base width. This effect limits the output impedance of bipolar amplifiers. The Gummel-Poon model, an extension of the Ebers-Moll model, accounts for high-level injection and other second-order effects needed for accurate circuit simulation.
Field-Effect Transistors
Field-effect transistors control current using an electric field rather than a base current. The MOSFET, or metal-oxide-semiconductor field-effect transistor, is the most important device in modern electronics. It consists of two heavily doped regions, the source and drain, separated by a channel region, with a thin insulating oxide layer and a gate electrode above the channel.
In an n-channel MOSFET, applying a positive voltage to the gate creates an inversion layer of electrons at the silicon surface, forming a conducting channel between source and drain. The threshold voltage is the minimum gate voltage needed to create the channel. Above threshold, the drain current increases with the square of the gate voltage in saturation and linearly with both gate voltage and drain voltage in the triode region.
MOSFET Scaling
Moore’s Law described the exponential scaling of MOSFET dimensions that has driven semiconductor progress for decades. As transistors shrink, they switch faster, consume less power per device, and cost less to manufacture. But scaling has encountered fundamental physical limits. Gate oxide thickness is now only a few atomic layers, and leakage currents through the oxide increase exponentially as it thins.
Modern transistors use strained silicon to increase carrier mobility, high-k dielectric materials to reduce gate leakage while maintaining capacitance, and metal gates to eliminate polysilicon depletion effects. FinFET structures raise the channel as a vertical fin surrounded by the gate on three sides, providing better electrostatic control and reducing leakage. Gate-all-around transistors represent the next step, surrounding a horizontal nanowire channel with the gate on all sides.
Integrated Circuit Fabrication
Integrated circuits are fabricated on thin wafers of single-crystal silicon, typically 200 or 300 millimeters in diameter. The fabrication process involves hundreds of steps performed in cleanrooms where particle contamination is strictly controlled.
Photolithography patterns each layer of the device. A photosensitive resist is applied to the wafer, exposed through a mask containing the pattern for that layer, and developed to leave resist only on the regions to be protected. Etching removes material from unprotected areas. Deposition adds new layers of conductors, insulators, or semiconductors.
The number of mask layers has increased with each technology generation. A modern 5-nanometer process uses 80 or more mask layers, each requiring precise alignment to previous layers. The cost of a fully equipped fabrication facility now exceeds $10 billion, making semiconductor manufacturing one of the most capital-intensive industries in the world.
Advanced Device Structures
Beyond the basic MOSFET, advanced devices continue to push performance boundaries. Silicon-germanium heterojunction bipolar transistors achieve cutoff frequencies exceeding 500 GHz, making them essential for millimeter-wave communication and high-speed instrumentation.
High-electron-mobility transistors use heterojunctions between gallium arsenide and aluminum gallium arsenide to create a two-dimensional electron gas with exceptionally high mobility. HEMTs are the standard for low-noise amplifiers in satellite communication, radar, and radio astronomy.
Power semiconductor devices including IGBTs and power MOSFETs have evolved to handle ever-higher voltages and currents. Silicon carbide devices are replacing silicon IGBTs in applications from electric vehicle traction drives to renewable energy inverters, offering higher voltage ratings, higher switching frequencies, and better thermal performance.
Frequently Asked Questions
What is Moore’s Law and is it still valid?
Moore’s Law is the observation that the number of transistors on a chip doubles approximately every two years. While the traditional scaling of planar transistors has slowed, the trend continues through new device architectures like FinFETs, improved materials, advanced packaging, and chiplet integration. The pace has slowed from 18-month doubling to roughly 2.5 to 3 years, but progress continues.
Why is silicon the dominant semiconductor material?
Silicon dominates because of its abundant supply, the exceptional quality of its native oxide (silicon dioxide) for gate insulation, its wide band gap that enables operation at elevated temperatures, the mature manufacturing infrastructure developed over decades, and its compatibility with both analog and digital circuits. No other material combines all these advantages.
What is the difference between a bipolar and a MOSFET transistor?
A bipolar transistor is current-controlled — a base current controls the collector current. It has high transconductance and drives high currents efficiently but requires continuous base current. A MOSFET is voltage-controlled — the gate voltage controls the drain current. MOSFETs have extremely high input impedance, are easier to fabricate in dense integrated circuits, and consume no gate current in steady state.
How are integrated circuits manufactured?
ICs are manufactured through photolithographic patterning of multiple material layers on a silicon wafer. Each layer involves coating with photoresist, exposing through a mask, developing, etching, and depositing or growing the next material. Modern processes have 80-plus mask layers with feature sizes measured in nanometers. After fabrication, wafers are tested, diced into individual chips, assembled into packages, and tested again before shipment.